1. Field of the Invention
The present invention relates to a double-faced image formation system that forms images on both sides of a sheet, specifically to a type of double-faced image formation system that forms images on one side of a recording medium, and then forms images on the other side of the same recording medium.
2. Description of the Related Art
Conventionally, an image formation system is known, in which are disposed in parallel plural image formation units provided with image carriers (for example, photosensitive drums) on which toner images are formed and retained, an intermediate transfer belt is provided which moves to circulate along the direction of each image formation units being arrayed, the images of each color components (for example, yellow, magenta, cyan, black) formed by each image formation units are sequentially primarily transferred onto the intermediate transfer belt, and thereafter the images superposed on the intermediate transfer belt are secondarily transferred onto a sheet in a lump. Of this type of image formation system, a double-faced image formation system that forms images on both sides of a sheet is known, in which, when the double face mode is selected, the images superposed on the intermediate transfer belt (front face images) are secondarily transferred onto the front face of the sheet, the front face images are fixed by a fixation device, after the sheet is reversed by means of a sheet reversing conveying mechanism, the sheet is reconveyed to a secondary transfer unit, the images superposed on the intermediate transfer belt (rear face images) are secondarily transferred onto the rear face of the sheet, and the rear face images are fixed by the fixation device.
And, the fixation device provides a pair of fixation members that rotate in contact with each other (for example, a heating roll and a pressure roll to press in contact with the heating roll), and passes the sheet through a nipping area between the fixation members to thereby fix the yet-to-be-fixed toner images on the sheet. A conventional fixation device is known which provides an oil coating applicator to apply oil as a lubricant on the surface of the heating roll in order to prevent the so-called offset phenomenon that the toner on the sheet transfers to the heating roll (refer to Patent Reference 1).
[Patent Reference 1]
Japanese Published Unexamined Patent Application No. 2000-267381
In the double-faced image formation system using this sort of fixation device, if only the surface of the heating roll has the oil applied, since the heating roll and the pressure roll rotate in contact with each other before the fixation processing, the surface of the pressure roll will have the oil applied indirectly. Accordingly, when fixing the front face images (yet-to-be-fixed toner images) secondarily transferred on the front face of the sheet, there occurs a situation that the oil is transferred and adhered on the rear face (the face that comes in contact with the pressure roll) of this sheet. At that moment, the oil is electrified to the positive or negative polarity on the energy level of molecule in many cases. Under such a situation, if the sheet is reversed and the rear face images are secondarily transferred on the rear face of this sheet, the oil electrified to the reverse polarity to the toner will be transferred to the surface of the intermediate transfer belt from the rear face of the sheet by the function of the secondary transfer field. However, the oil is difficult to transfer to the area (image area) where the toner images are present on the intermediate transfer belt by the influence of the secondarily transferred toner; accordingly, the oil will transfer only to the area (non-image area) where the toner images are not present on the intermediate transfer belt.
Now, supposing a case that one double-faced image is sequentially formed to 1000 sheets, for example, the oil hardly transfers to the area corresponding to the image area, which exists on the same position on the intermediate transfer belt, but the oil sequentially transfers only to the area corresponding to the non-image area. As the quantity of the oil transferred to the intermediate transfer belt increases, the surface energy of the oil-adhered area on the intermediate transfer belt remarkably varies, which influences the transfer performance of the toner images. To explain this in detail, after executing the double-faced image formation sequentially multiple times as to one image, when an image different from the same image, especially a half-tone pattern image is formed on the whole area, the transfer efficiency in the oil adhered area (corresponding to the non-image area) on the intermediate transfer belt is enhanced in comparison to the other area. As the result, the rear face image formed on the formation of the double-faced image directly before appears faintly in a negative image in the half-tone image next formed, thus creating the so-called ‘oil ghost’ phenomenon.
The above Patent Reference 1 discloses a technique that installs a static eliminator in the sheet reversing conveying mechanism, eliminates static electricity from oil adhered on the front and rear faces of a sheet with images formed on the front face thereof, and thereby restrains the transfer and adhesion of the oil to the intermediate transfer belt.
However, even in the case of using the technique disclosed in the Patent Reference 1, there is a possibility that the oil adhered on the sheet will be transferred and adhered to the intermediate transfer belt, which proved that the technique was yet insufficient to prevent the above oil ghost as the measure. To be more concrete, the technique disclosed in the Patent Reference 1 definitely eliminates static electricity from the electrified oil to thereby remove an electrostatic factor. However, since the oil adhered to the sheet comes directly in contact with the intermediate transfer part (image retaining conveyer), it is confirmed that a physical adhesive force makes the oil transfer and adhere from the sheet to the intermediate transfer part (image retaining conveyer).